As a technique for realizing a large capacity communication network, a wavelength division multiplexing (WDM) has spread widely. WDM is capable of transmitting a plurality of signals using a plurality of wavelength channels.
A reconfigurable optical add-drop multiplexer (ROADM) is implemented in each node of a WDM network that transmits a WDM optical signal. The ROADM includes a wavelength selective switch (WSS) and individually processes each optical signal multiplexed in the WDM optical signal. Specifically, the ROADM drops an optical signal of a desired wavelength from the WDM optical signal. Further, the ROADM adds an optical signal to an idle channel of the WDM optical signal.
In the WDM network, while being transmitted from a transmission station to a reception station, an optical signal of each wavelength channel passes through one or a plurality of ROADMs. At this time, the optical signal passes through the WSS in each node. When a WSS passes an optical signal in a specified wavelength channel, the WSS provides a pass band to the specified wavelength channel. A width of the pass band is determined, for example, based on a spacing of a wavelength grid of the WDM network.
However, in a case in which an optical signal passes through a plurality of WSSs, a pass band of an optical transmission path for the optical signal maybe narrowed. This phenomenon may be called pass-band narrowing (PBN). Further, the pass-band narrowings differ from each other in accordance with a route between a source node and a destination node. That is, the transmission characteristics of the optical transmission path differ from each other in accordance with the route between the source node and the destination node. Accordingly, to improve communication quality, it is requested to monitor the transmission characteristics of the optical transmission path for respective routes.
As a related technology, a device and a method are proposed for monitoring a transmission wavelength band of a wavelength tunable optical filter (for example, Japanese Laid-open Patent Publication No. 2014-143614). A method for monitoring a pass band of a WSS is proposed (for example, WO2013/140493). Further, a method for calculating a shape of a pass band of an optical transmission path is proposed (for example, the following documents 1 and 2).    Document 1: Guoxiu Huang et al., Pass-band shape monitor for minimizing impact of signal filtering in cascaded ROADMs, Asia Communications and Photonics Conference 2015, AM1E.4    Document 2: Cibby Pulikkaseril et al., Spectral modeling of channel band shapes in wavelength selective switch, OPTICS EXPRESS, Vol. 19, No. 9, pp. 8458-8470, 2011
The transmission characteristics of the optical transmission path are obtained by measuring a power of probe light. For example, a transmitter transmits probe light. The probe light is transmitted to a receiver through the optical transmission path. Here, a wavelength of the probe light is swept in a wavelength range of a target wavelength channel. Note that a transmission power of the probe light is constant. The receiver measures a received power of the probe light. Then measured values of the received power are plotted for each wavelength and thereby a pass-band shape (PNS) is detected with respect to the target wavelength channel.
However, an optical amplifier implemented in each relay node generates optical noise. For example, an erbium-doped fiber amplifier (EDFA) generates amplified spontaneous emission (ASE) noise. Therefore, when the probe light is transmitted from the transmitter for monitoring the transmission characteristics of the optical transmission path, the receiver detects the probe light and the ASE noise. Thus, a measurement value of the received power of the probe light includes an error caused by the noise. Accordingly, in an environment in which optical noise (particularly, ASE noise) is generated in the optical transmission path, it is difficult to accurately monitor the transmission characteristics of the optical transmission path.